Quantum chemical investigation of A2LiBiI6 perovskites with Na, K, and Rb for photocatalytic water-splitting application

Perovskite materials have received a lot of attention due to their distinctive structural, electronic, and optical characteristics, particularly in photocatalytic water splitting applications. This work aims to explore structural, electronic, optical, elastic, and mechanical properties of cubic-phase A2LiBiI6 (A = Na, K, Rb) double perovskites using DFT within the GGA-PBE framework. All compounds exhibit negative formation energies (−1.45, −1.14, and −1.07 eV) along with the tolerance factors ranging from 0.820 to 0.896, which fall within the perovskite stability domain. In the phonon dispersion analysis of all compounds, no imaginary frequencies appeared, confirming their dynamic stability. The materials possess a bandgap 1.891–2.014 eV (GGA-PBE) and 1.896–2.038 eV (YS-PBE0), appropriate for visible light absorption, with band gap energies increasing systematically with A-site ionic radius (Na < K < Rb). Dielectric function analysis reveals stable dispersion across the visible spectrum, with static dielectric constants ranging from 3.9 to 4.7, indicating a high degree of polarizability. Mechanical assessments, including positive shear modulus (G) values and Pugh’s ratio (B/G > 1.75), confirm their ductile and stable nature. This work lays the groundwork for designing cost-effective photocatalytic materials for hydrogen evolution reaction (HER).